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Langmuir 1991, 7, 2911-2916
Preparation and Properties of Uniform-Coated Colloidal Particles. 6. Titania on Zinc Oxide? Manuel Ocaiia,S Wan Peter Hsu, and Egon Matijevie' Department of Chemistry, Clarkson University, Potsdam, New York 13699 Received February 28,1991 Nearly spherical particles of zinc oxide were prepared and coated with uniform layers of amorphous titanium dioxide by hydrolysis and condensation of titanium butoxide in ethanolic solutions containing the preformed cores. The thickness of the coating could be altered by adjusting the concentrations of the reactants (titanium alkoxide and water) and the amount of added ZnO. On calcination >700 "C the shells of Ti02 react with the ZnO cores forming zinc titanates of various stoichiometries (a-ZnzTiO4,ZnzTi308, and ZnTiOs). The optical properties of the solids as prepared and after calcination at several temperatures were measured and evaluated in terms of the Mie scattering coefficients.
I. Introduction The deposition of uniform layers of a given material on solids of a different composition is usually carried out in order to modify or improve properties of the latter, e.g., chemical, magnetic, optical, etc.'+ Such powders are also of economic interest when a process involves precious reactants, which can be coated on inexpensive cores. Finally, if a material cannot be prepared in particles of a desired shape (e.g. rodlike), one may deposit it on cores of a different composition but of the required morphology. Mixed ZnO-Ti02 solids have been used as gas sensors,1° dielectric ceramicell and in the paint industry.12J3 For many of these applications, and specifically for their use as white pigments, powders consisting of uniform spherical particles are highly desirable, since their optical characteristics can be better controlled and reproduced. In this work, the preparation and properties of nearly spherical ZnO particles coated with uniform layers of Ti02 are described. The latter was deposited by hydrolysis of titanium butoxide onto ZnO cores, obtained by a modified method described earlier,14 dispersed in ethanolic solutions. The effects of the concentration of reactants (butoxide and water) and ZnO cores on the coated layers were studied in detail. Finally, the optical properties in the visible region, of the powders as prepared and after calcination at different + Supported by a contract from the XMX Corp., Burlington, MA. On leave from Instituto de Ciencia de Materials,Madrid, Spain. (1) Parfitt, G. D.; Ramsbotham, J. J. Oil Colour Chem. Assoc. 1971,
*
54, 356. (2) Cornell, R. M.; Posner, A. M.; Quirk, J. P. J. Chem. Technol. Biotechnol. 1980. 30. 187. (3) Hitach; Mhell, Ltd. Jpn Kokai Tokkyo Koho, JP 58 161708, September 26, 1983. (4) Ube Industries, Ltd. Jpn Kokai Tokkyo Koho, JP 59 213626, December 3, 1984. (5) Frank, A. J.; Willner, I.; Goren, Z.; Degani, Y. J . Am. Chem. SOC. 1987,109,3568. (6) Morrison, C.; Kiwi, J. J . Chem. SOC., Faraday Trans. I 1989,85, 1043.
(7) Tanabe, K.; Ishiya, C.; Matsuzaki, I.; Ichikawa, I.; Hattori, H. Bull. Chem. SOC. Jpn. 1972,45, 47. (8) Boor, J.; Bauer, R. S. J. Appl. Polym. Sci. 1974,18, 3699. (9) Villa, P.; Del Piero, G.; Uetti, L.; Garagiola, F.; Mologni, G.; Tronconi, E.; Pasquon, I. Appl. Catal. 1987, 35, 47. (10) Matsushita Electric Industrial Co., Ltd. Jpn Kokai Tokkyo Koho, JP 82 80549, May 20, 1982. (11) Berton, J. F.; Roelandt, B. Bull. Soc. Fr. Ceram. 1972, 94, 51. (12) Payne, H. F. In Organic Coating Technology; John Wiley: New York, 1961; Vol. 11. (13) Gilligan, J. E.; Zerlant, G. A. J . Eng. Ind. 1973, 95, 1065. (14) Chittofratti, A.; MatijeviE, E. Colloids Surf. 1990, 48, 65.
0743-7463/91/2407-2911$02.50/0
temperatures (1900"C), were determined and the results evaluated in terms of the Mie scattering coefficients.15J6
11. Experimental Section Materials. Zinc nitrate (Baker),sodium dodecyl sulfate (SDS, Biochemical),triethanolamine (TEA, Mallinckrodt),titanium(IV)butoxide (TBOT, Aldrich), and ethyl alcohol (200 proof, Pharmco)were used without further purifications. A commercial sample of rutile (RLP2 type), the diameters of which ranged from 0.2 to 0.3 pm, was supplied by the WKP Company (Stuttgart, Germany). Preparation of Particles. 1. Zinc Oxide. Nearly spherical, crystalline ZnO cores were prepared by a procedure described earlier,14introducing some modifications. Solutions0.0015 mol dm-3 in Zn(NOs)z, 0.1 mol dm-3in triethanolamine (TEA), and 0.01 mol dm" in sodium dodecyl sulfate (SDS)were aged under stirring for 1 h, in closed glass bottles (500 cm3),and placed in a bath thermostated at 90 "C. The stock solutions of Zn(NO& (0.05 mol dm-3),TEA (1 mol dm-9, and SDS (0.1 mol dm-9 were filtered through 0.22 pm pore size Millipore membranes, and appropriate volumes were then mixed and diluted with doubly distilled water to achieve the final concentrations. The sequence followed in the addition of solutions was Zn(NO&, SDS, and TEA. On termination of aging, the resulting dispersions were cooled in an ice-water bath and centrifuged and the precipitates were washed several times with doubly distilled water, until no precipitation of BaSOd could be detected in the supernatant solution with BaC12. Finally, the powders were dried at 50 "C for 2 days. 2. Coated Particles. Coating procedure consisted of controlled hydrolysis of ethanolic solutions of titanium butoxide (TBOT) in the presence of ZnO powder. For this purpose a weighed amount of ZnO particles was dispersed in an ultrasonic bath in 10 cm3of ethanol, to which an aliquot of doubly distilled water was added. Titanium alkoxidewas dissolved in a separate volume of ethanol (10 cm3)and this solution was rapidly added to the above dispersion. The mixture was then aged for 30 min in test tubes, tightly closed with Teflon-lined caps, placed in an oven preheated at 100 O C . The concentrations of TBOT and water, as well as of the amount of ZnO, were varied in order to study their effects on the surface coating. After aging, the dispersions were cooled in an ice-water bath and then diluted with ethanol to stop the hydrolysis. The solids were separated by centrifugation and washed 3 times with ethanol (to prevent the hydrolysisof the residual titanium alkoxide) and, finally, 6 times with doubly distilled water. The purified powders were dried at 50 "C for 2 days and stored in a desiccator. 3. Analyses. The morphology of the particles was examined with a JEM-1200 transmission electron microscope (TEM) and (15) Toon, 0. B.; Ackerman, T. P. Appl. Opt. 1981,20,3657. (16) Bohren, C. F.; Huffman, D. R. In Absorption and Scattering of Light by Small Particles; John Wiley: New York, 1983.
0 1991 American Chemical Society
2912 Langmuir, Vol. 7,No.12,1991 their size distributions were determined from electron micrographs, as well as by light scattering, using the polarizationratio method.I7 The metal contents (zinc and titanium) of the powders were analyzed by a Sargent XVI polarograph using a droppingmercury electrode in combination with a standard calomel electrode. Weighed amounts of the solids (12-30 mg) were dissolved in 2 cm3of concentrated HCl(37 w t %) and diluted to 20 cm3with doubly distilled water. Oxalic acid (1mol dm-3) and ammonia buffer (2 mol dm-3 NH40H and 2 mol dm-3 NH4C1) were used as a supporting electrolyte for Ti(1V) and Zn(II), respectively. The polarographic waves of titanium and zinc in these solutions were measured for the unknown solutionsand comparedto those from the corresponding standard solutions. The experimental error of this method was estimated to be less than 5 % . The calcinationof coated particles was carried out by keeping the samples for 1 h in a tubular furnace, preheated at desired temperatures ranging from 200 to 900 "C. The phase transformation of solids on heating was followed by X-ray diffraction (Siemens D500 instrument with Cu Ka radiation), infrared spectroscopy (Perkin-Elmer 1430 photometer), differential thermal (DTA) (Perkin-Elmer 1700),and thermogravimetric (TGA) (Perkin-Elmer 7) analyses. The last measurements were done under nitrogen with a heating rate of 10 "C min-l. A Perkin-Elmer Lambda 3 UV/VIS spectrophotometer equipped with an integrating sphere attachment was used for the optical (opacity) evaluation of the solid samples. For this purpose, 0.25 g of powder was dispersed in 100 cm3of distilled water in an ultrasonicbath, and each dispersionwas then filtered through separate Millipore membranes, HABP (black) and HAWP (white). After filtration, the powder on the membrane was uniformly wetted with a few drops of a glycerol-water mixture (2:l) and stored in a desiccator overnight. The refractive index of the liquid mixture, n, 1.43, was measured with an Abbe refractometer. The values of the luminous reflectance ( Y % )for the black and white membrane are 0.04 and 0.99, respectively, using a MgO plate as a reference. The apparent opacitiesof the powders were then obtained by dividing the Y% value of the powder deposited on a black membraneto that of the samepowder deposited on a white membrane. The values of Y% for the reflectance spectrawere calculated accordingto the CIE Standard Colorimetric System for a light source C.18
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Ocafia et al.
Figure 1. Transmission electron micrograph (TEM) of ZnO particles obtained by aging at 90 "C, for 1 h, under stirring, solutions0.0015 mol dm-3in Zn(NOJ2,O.l mol dm-3in TEA, and 0.01 mol dm-3 in SDS. ZnO
111. Results
Preparation and Composition. A transmission electron micrograph (Figure 1)illustrates the nearly spherical particles of ZnO used as cores, prepared under the described conditions. The corresponding size distributions as evaluated by electron microscopy (histogram)and light scattering (curve) give a modal diameter is 0.36 pm with a standard deviation of 0.18 (Figure 2). The X-ray diffraction pattern of this powder showed characteristics of crystalline zincite.19 The coating of ZnO particles with titania was achieved under a limited set of conditions. Thus, uniform layers of Ti02 were obtained when 1.5 mg of the cores was dispersed in a solution of 0.015 mol dm-3 TBOT containing water in the concentration range of 0.27-0.54 mol dm-3. At higher concentrationsof the alkoxide (>0.025 mol dm-3) aggregation took place, whereas a t lower concentrations (250 "C,the additional decrease in the sample weight is less than 1% for all these solids. The exothermic peak a t -400 "C in the DTA curve for the hydrous titania (Figure 6, I) corresponds to the crystallization of amorphous Ti02 into anatase, as confirmed by the X-ray diffraction. The transformation of anatase into rutile could not be clearly detected by the
Langmuir, Vol. 7, No. 12, 1991 2915
Uniform Coated Colloidal Particles DTA; however, the X-ray diffraction indicated that such a process took place between 700 and 800 "C. The X-ray analysis of sample A heated a t 700 "C shows only zinc oxide as the crystalline component, while the X-ray pattern of the same powder calcined a t 900 "C (Figure 4,II) is characteristic of both ZnO and ~r-Zn2Ti04.l~ The transmission electron micrographs of the particles treated a t both these temperatures show nodistinct surface layers as illustrated in Figure 3C for sample A a t 700 "C. It would seem that zinc titanate is formed a t both temperatures, but it is amorphous at 700 "C and crystalline a t 900 "C. In contrast, sample B yielded crystalline zinc titanates already a t 700 "C, which were identified by X-ray analysis as a mixture of a-Zn2Ti04, Zn2Ti308, and ZnTi0319 in addition to ZnO (Figure 4, 111). The last cohponent essentially disappeared on calcination a t 900 " C, which was accompanied by a decrease in the intensity of the peaks characteristic of ZnTiO3 (Figure 4,IV). The TEM pictures of particles treated a t 700 and 900 "C showed no distinct coated layers, while a t the higher temperature, structural subunits became visible (Figure 3D). No sintering took place on calcination of these powders up to 900 "C. The shallow peaks on DTA curves between 450 and 800 "C for sample A and between 400 and 700 "C for sample B (Figure 6, I) may be due to the formation of the mentioned zinc titanates. The infrared spectra of the calcined solids also corroborate the described transformations. The spectral features of sample B calcined a t 900 "C (Figure 5, 111) agree, in general, with those previously reported for Z112Ti04~land Zn2Ti308T2although some bands are slightly shifted with respect to those of the pure phases. A change in the spectrum is already noted when sample B is calcined a t 700 "C, with some differences in the position of the bands, probably due to the contribution by both pure ZnO and ZnTiO3. The latter phase is likelyresponsible for the band a t 330 cm-l, since it disappears after heating a t 900 "C. Finally, no changes were detected in the spectrum of sample A calcined at 700 "C (Figure 5,111. However, this sample heated a t 900 "C shows a band corresponding to ZnO (440 cm-') along with two shoulders at -565 and -700 cm-l, which indicate the presence of a-ZnnTi04. Optical Characterization. Optical reflectance spectra of samples A and B, heat treated a t different temperatures, are illustrated in Figure 7, along with the spectra of ZnO and TiO2, as references. Table I1 summarizes the size distribution (in terms of the diameter, d, and the size parameter, ao) and optical properties (refractive index and opacity) of powdered samples A and B before and after heat treatments, as well as of zinc oxide and titania particles. The light scattering procedure" yields the refractive index, nLs, of the investigated powder, in addition to particle modal diameter and the size parameter (ao). Since the size distribution is approximately the same for all powders studied, the ratios Q,,,/dp (QScabeing the scattering coefficient and p the density in g ~ m - are ~) proportional to the measured opacities. By use of titania as reference (1003'% 1, the normalized experimental values of opacity were then compared to a series of theoretical values of Qsca/dp,assuming a concentric sphere for sample A with a coating thickness amounting to 35% of the particle radius and a homogeneous sphere for sample B. The (21) Keramidas, V. G.; Deangelis, B. A,; White, W. B. J. Solid State Chem. 1975, 15, 233. (22) Yamaguchi, 0.;Morimi, M.; Kawabata, H.; Shimizu, K. J. Am. Ceram. SOC.1987, 70,C-97.
inn,
1
Yz
P
600
500
400
WAVELENGTH / nm Figure 7. Optical reflectance spectra of ZnO, Ti02 (-), of samples A and B as prepared (- - -), and of the coated particles after calcination at 700 "C (-. -) and 900 "C (. e).
results listed in Table 11show that the calculated values of Qsca/dp, normalized with respect to titania, are in a good agreement with those obtained from the opacity measurements.
IV. Discussion
It is well-known that metal alkoxides readily hydrolyze in the presence of water, which, by a proper adjustment of the concentrations of the reagents, temperature, and pH, yields "monodispersed" particles of metal o ~ i d e s . ~ ~ - ~ ~ In this work, such a procedure was applied to deposit uniform layers of Ti02 over ZnO particles. The surface coatings may be formed either by surface nucleation and growth or by heterocoagulation of preformed coating material with the core particle^.^^ The uniformity of the surface shell in the last process depends not only on the rate of precipitation of the coating material but also on the surface charges of the two kinds of particles which undergo heterocoagulation. As a rule, such surface layers are not as smooth as those formed by surface nucleation, and in many cases, dispersions of mixtures of particles are produced. In this work, all coated particles consisted of a smooth surface, indicating the dominance of the surface nucleation and growth process. The thickness of the shells could be altered either by varying the content in water, which controls the yield of the precipitated TiOz, or by changing the amount of core particles; the higher the concentration of cores, the thinner is the layer. A similar trend has been previously reported for hematite particles covered with chromia28 and ~ t t r i a . ~ ~ It has been amply reported that ZnO reacts with Ti02 at temperatures higher than 600 "C to produce zinc titanates of various stoichiometries, e.g. a-ZnzTi04, Zn3~
~~~
(23) Stober, W., Fink, A.; Bohn, E. J . Colloid Interface Sci. 1968, 2 6 , 62. (24) Barrineer. E. A.: Bowen. H. K. J . Am. Ceram.SOC.1982.69. C-199. (25) Ogihari, T.; Ikemoto, T.; Mizutani, N.; Kato, M.; M i k a i , Y. J . Mater. Sci. 1986,21, 2771. (26) Ogihara, T.; Mizutani, N.; Kato, M. J . Ani. Ceram. SOC.1989,72, 421. (27) Gherardi, P.; MatijeviC, E. J . Colloid Interface Sci. 1986,109,57. (28) Garg, A.; MatijeviC, E. Langmuir 1988, 4 , 38. (29) Aiken, B.; MatijeviC, E. J. Colloid Interface Sci. 1988, 126, 645.
2916 Langmuir, Vol. 7, No. 12, 1991
Ocaiia et al.
Table 11. Particle Size Distribution and Optical Properties of ZnO, TiOz, and Coated Particles, before and after Heat Treatments ~
~~
normalized powder zinc oxide sample A original 700 OC 900 O C sample B original 700 "C 900 O C titania
Qw./dP, % 37
0.18
nLs 1.8
0.37 0.34 0.32
0.16" 0.15" 0.2P
1.8 (1.9)b 1.9 (2.0)b 2.0 (2.2)b
0.41 0.42 0.51
47 48 58
45 49 60
0.44 0.36 0.34 0.30
0.19 0.17 0.22 0.25
1.9 2.3 2.2 2.8
0.45 0.65 0.63 0.88
51 74 71
50 77 73 100
00
opacity 0.34
opacity, % 39
particle diameter, pm 0.36
100
Standard deviation, obtained from electron micrograph histograms. Assigned values of refractive indices of the core, n,, and the shell n, (in parentheses).
TizOs, and ZnTi03.22y3+32Several phase diagrams have been reported for this which were found to depend on the source of Ti02 used (titanic acid, anatase, or rutile). In agreement with these diagrams, only Zn2TiO4, along with an excess of ZnO, was identified after calcining sample A a t 900 "C. The reflectance measurements (Figure 7) showed that the opacity increased from 0.42 to 0.51 when the powder was heated a t 900 "C. This effect can be attributed to the formation of crystalline ZnzTiO4, the refractive index of which is 2.2 (Table 11). The opacity of the powder calcined a t 700 "C increased only a little as compared to the original sample, indicating that the crystallization process was still incomplete. The chemical changes observed with the sample B were not in accordance with the phase diagrams. While in the present case calcination a t 700 "C yielded a-ZnzTiOr,Zn2Ti308, ZnTiO3, and ZnO, solids previously prepared by heating a t 700 "C a powdered mixture of ZnO and TiO2, a t the same molar ratio as sample B, consisted only of ~~-ZnTi204~O or of a-ZnTi204 and ZnzTi30~,~l along with (30) Dulin, F. H.; Rase, D. E. J. Am. Ceram. SOC.1960, 43, 125. (31) Bartram, S. F.; Slepetys, R. A. J. Am. Ceram. SOC.1961,44,493. (32) Watanabe, A.; Haneda, H.; Moriyoshi, Y.; Shirasaki, S.; Yamamura, H. J. Mater. Sci. 1989, 24, 2281.
pure phases (ZnO and TiO2). Bartram and Slepetys31 explained the formation of ZnTiO3 on the basis of crystallographic studies; accordingly, this phase should only be produced from rutile, or other sources of Ti02 which readily transform into rutile at 750 "C. The amorphous Ti02 obtained in this work in the "blank" experiments was found to crystallize into rutile a t 700800 "C. The significant increase in the luminous reflectance (Figure 7b) of sample B heat-treated a t 700 "C is due to the formation of zinc titanates. Finally, it has been previously reported30 that ZnTiOa dissociates a t 900 "CgivingZnzTiO4and ZnO. In contrast, this work indicates that a t this temperature ZnTiO3 reacts with ZnO to form Zn2TiO4, which causes a decrease in the opacity of the powder.
Acknowledgment. The authors are indebted to Professor Petr Zuman for the use of the polarographic equipment and for constructive discussions. Registry No. ZnO, 1314-13-2; TiOz, 13463-67-7; ZnzTiOr, 12036-69-0; ZnzTi308, 12037-09-1; ZnTiO3, 12036-43-0; TBOT, 5593-70-4.
